Coronary reserve: Concept and physiological variations

When myocardial oxygen consumption (MVO2 is constant, coronary blood flow is largely independent of coronary perfusion pressure within the limits of autoregulation. The relationship is described by a slightly rising plateau, the level of which depends on the value of MVO2. After maximum arteriolar vasodilatation (hyperaemia) coronary blood flow is no longer autoregulated and varies linearly with perfusion pressure. The difference between the coronary blood flow corresponding to the autoregulation plateau under baseline conditions (Qbasal), and the coronary blood flow after maximum vasodilatation (Qmax) is the coronary flow reserve (CFR), which is generally expressed as the Qmax/Qbasal ratio, equal to 4 to 5 in normal subjects. A transient increase in the perfusion pressure within the autoregulation range, causes little or no change in the CFR, as basal flow and peak flow increase proportionally. The CFR decreases progressively with increasing heart rate. This decrease is partly due to an increase in basal flow, secondary to an increase in MVO2, whereas peak flow remains unchanged. An acute increase in left ventricular preload induces a decrease in CFR, due to an increase in basal flow secondary to increased wall stress and therefore increased MVO2, whereas peak flow remains unchanged. Experimentally, peak flow is not affected by an acute increase in contractility.